Fix #352: separate runlevel S from runlevel 0

Due to an old design decision runlevel S was encoded internally as '0',
meaning it was the same as halt/poweroff.  If you want to run scripts at
system shutdown this was less then ideal since it meant your scripts
also ran at bootstrap.

This change is quite invasive.  It introduces INIT_LEVEL (10) as the
value for runlevel S, meaning all code that parses and/or evaluates
anything for runlevel 0/S was affected and had to be reworked.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/sm.c

@@ -86,10 +86,10 @@ static char *sm_status(sm_state_t state)
  */
 static void nologin(void)
 {
-	if (runlevel == 1 || runlevel == 0 || runlevel == 6)
+	if (runlevel == 1 || IS_RESERVED_RUNLEVEL(runlevel))
 		touch(FINIT_NOLOGIN_PATH);
 
-	if (prevlevel == 1 || prevlevel == 0 || prevlevel == 6)
+	if (prevlevel == 1 || IS_RESERVED_RUNLEVEL(prevlevel))
 		erase(FINIT_NOLOGIN_PATH);
 }